Anions are major contaminants in water treatment, desalination and related environmental remediation processes. Anions such as nitrate (NO3−), perchlorate (ClO4−), and sulfate (SO42−) are having a major impact on water quality throughout the world. NO3− is an acute water contaminant that reduces the ability of red blood cells to carry oxygen when ingested. It is often found in agricultural run-off, in industrial or municipal wastewater, and in lakes, rivers and coastal water systems, where it may cause hypoxia (i.e., oxygen deficiency). ClO4− is a water-soluble anion that has been widely used in the manufacturing of explosives, mines and solid propellants for pyrotechnic devices, rockets and missiles. Sulfate is a product of burning coal, and sudden exposure to high doses may cause diarrhea in animals and infants. Both nitrate and sulfate are also commonly found in fertilizers, manure, and waste discharged from septic tanks.
Anion removal is a key component of water treatment processes. Membrane separation technologies such as reverse osmosis (RO), nanofiltration (NF) and electrodialysis (ED) may not be cost-effective at recovering anions from aqueous solutions. Although bioreactors can effectively reduce NO3− to N2 gas under anaerobic conditions, regulatory and public acceptance may limit their utilization in drinking water treatment.
Ion exchange (IX) may be used in the art for treating water contaminated by a variety of anions including ClO4− and NO3−. However, drawbacks of IX include limited selectivity, low efficiency and environmental impact (e.g., brine management and disposal). In general, IX is a heterogeneous solid-liquid process that involves ion diffusion through the resin pores followed by an exchange reaction; therefore, ClO4− and NO3− binding to IX resins generally require longer equilibration time. There are many types of IX resins; as an example, the perchlorate binding capacity of one brand, the commercially used disposable AMBERLITE PWA2 IX resin from Purolite, is 65 mg/g.
Two commonly used IX resins for perchlorate and nitrate removal are: (i) non-selective polyacrylic anion-exchange resins and (ii) selective polystyrenic anion-exchange resins. Both resins contain Cl− exchange sites. The non-selective resins preferentially sorb divalent anions such as SO42−; whereas the anion-selective resins have much higher affinity for poorly hydrated anions such as ClO4−. Resin regeneration/reuse and brine management/disposal are two issues that have a significant impact on the effectiveness, cost and environmental acceptability of IX systems used to treat water contaminated by ClO4− and NO3−. In the case of perchlorate, the non-selective resins require frequent regenerations with brine (˜6-12 wt % NaCl solution) due to their low ClO4− capacity/selectivity. This generates a significant amount of wastes (e.g., brine and ClO4−) that need to be treated prior to disposal. Conversely, the ClO4− selective IX resins generally do not require frequent regenerations. However, even concentrated brine solutions (˜12 wt %) are not generally effective at regenerating the resins in this case because of their typically strong binding affinity for ClO4−. Because of this, spent ClO4− selective resins generally cannot be cost-effectively regenerated and are incinerated in most cases following a single use.
Given the drawbacks of present technology, better methods are needed for filtering water, or for separating various ions, particularly anions, from aqueous solutions.